In prior U.S. Pat. No. 5,685,693 of common assignee herewith, there is illustrated an industrial gas turbine having inner and outer shells. The inner shell has a pair of axially spaced circumferential arrays of radially outwardly projecting pins terminating in reduced sections having flats on opposite circumferential sides thereof. Generally, cylindrical sleeves project inwardly and about access openings in the outer shell and have threaded bolt holes extending in circumferential directions. Bolts extend through the holes to engage the flats on the sides of the pins. By adjusting the bolts, the inner shell is adjustable externally of the outer shell to locate the inner shell about the rotor axis. During turbine operation, the inner shells may expand or move out of roundness and concentricity with respect to the outer shell when the shells respond to thermal and physical loads. Since turbine efficiency is affected by the roundness and concentricity of the inner shell with respect to the outer shell, this allowed realignment of the shells without disassembling the turbine. Roundness and concentricity determine the gap between the turbine buckets (attached to the rotor) and the bucket shrouds (attached to the turbine shell) which in turn determines the amount of gas that bypasses the bucket. Since no work is extracted from this bypass gas by the bucket, gas turbine performance is inversely proportional to this clearance gap.
This problem was further addressed in U.S. Pat. No. 6,457,936, which is hereby incorporated by reference in its entirety, with an improved mounting arrangement between the inner and outer shells using support pins loaded only in the circumferential or tangential direction. The circumferentially spaced support pins are disposed through access openings in the outer shell and have projections received in recesses of the inner shell to engage the two shells in a manner that supports the inner shell against radial and circumferential movement relative to the outer shell and enables thermal expansion and contraction of the inner shell relative to the outer shell in radial and axial directions. By controlling the thermal expansion and contraction of the inner turbine shell, the clearance gap between the bucket tips and the shrouds is controlled during the operation of the gas turbine, resulting in improved efficiency.
Recent simulations of the support pins, however, show that concentricity and roundness of the inner turbine shell are affected by how the pins initially are gapped during assembly. Specifically, for an inner turbine shell radially supported by multiple support pins, a minimum of at least two pins must support the gravitational loading when the rotor engine is at rest. When the engine starts, the support pins counteract the applied torque generated by the nozzles carried by the inner shell. The two gravitational loaded pins are exposed simultaneously to the gravitational and counteracting torque loadings, which leads to loss of roundness and concentricity when the shells differentially expand during turbine operation. Moreover, one of the pins is exposed to a gravitational loading in the opposite direction as the counteracting torque loading. When the turbine runs, this pin and the next adjacent pin push the inner shell segment between them against each other. Consequently, when the segment of the inner shell expands during operation, this segment becomes pinched between the two pins, which causes the entire inner turbine shell to be eccentric and out of roundness. One way of addressing these problems is to relax the initial gaps of the support pins, but such configuration reduces turbine efficiency.
Simulations also show that the surface profile of the contact line between the pin and the inner shell affects the concentricity and roundness of the inner shell during turbine operation. Accordingly, there remains a need for a more advanced configuration arrangement between the inner and outer shells in advanced gas turbine design.